FIG. 1 shows a basic structure of a circuit of a pixel (pixel circuit) in an active matrix organic electroluminescence (hereinafter referred to as “EL”) display device, and FIG. 2 shows an example structure of a display device (display panel) and an input signal to the display device.
As shown in FIG. 1, the pixel circuit includes a selection thin film transistor (hereinafter referred to as “TFT”) 2 having a source or a drain connected to a data line Data and a gate connected to a gate line Gate, a driving TFT 1 having a gate connected to the drain or the source of the selection TFT 2 and a source connected to a power supply PVdd, a storage capacitor C which connects between the gate and the source of the driving TFT 1, and an organic EL element 3 having an anode connected to the drain of the driving TFT 1 and a cathode connected to a low voltage power supply CV.
As shown in FIG. 2, a plurality of pixel sections 14 each having the pixel circuit shown in FIG. 1 are placed in a matrix form, to form a display section, and a source driver 10 and a gate driver 12 are provided for driving each pixel section in the display section.
An image data signal, a horizontal synchronization signal, a pixel clock, and other drive signals are supplied to the source driver 10, and the horizontal synchronization signal, a vertical synchronization signal, and other drive signals are supplied to the gate driver 12. The data line Data in the vertical direction extends from the source driver 10 for each column of the pixel sections 14 and the gate line Gate in the horizontal direction extends from the gate driver 12 for each row of the pixel sections 14.
The gate line (Gate) extending along the horizontal direction is set to a high level so that the selection TFT 2 is switched on, and a data signal having a voltage corresponding to a display brightness is supplied to the data line (Data) extending along the vertical direction in this state so that the data signal is accumulated in the storage capacitor C. With this process, a drive current corresponding to the data signal accumulated in the storage capacitor C is supplied by the driving TFT 1 to the organic EL element 3, and the organic EL element 3 emits light.
The current of the organic EL element 3 and the amount of light emission are in an approximate proportional relationship. Normally, a voltage (Vth) at which a drain current starts to flow around a black level of the image is supplied between the gate and PVDD (Vgs) of the driving TFT 1. As an amplitude of the image signal, an amplitude which results in a predetermined brightness around a white level is used.
FIG. 3 shows a relationship of a current CV current (which corresponds to the brightness) flowing through the organic EL element 3 with respect to the input signal voltage (voltage on the data line Data) of the driving TFT 1. By determining a data signal (Data voltage) so that Vb is supplied as the black level voltage and Vw is supplied as the white level voltage, the amount of light emission in the organic EL element 3 can be controlled from black to white, and a suitable grayscale control can be applied. As is clear from FIG. 3, the input voltage (Data voltage) of the pixel and the current are not in a completely proportional relationship.
In consideration of this, as shown in FIG. 4, a γ correction circuit (γ-LUT) 16 (16r, 16g, and 16b) is provided so that the relationship between the image data and the brightness is linear. The image data signal is a signal which represents brightness for each pixel, and because the image data signal is a color signal, the image data signal includes image data signals rn, gn, and bn for each color. Therefore, three γ correction circuits 16r, 16g, and 16b are provided corresponding to the colors of R, G, and B, and γ-corrected image data signals Rn, Gn, and Bn are output from the three γ correction circuits 16r, 16g, and 16b. Therefore, image data signals Rn, Gn, and Bn are supplied to the source driver 10, and subsequently, to the data line Data and to the pixel sections 14 for R display, for G display, and for B display, respectively. As shown in the figures, the source driver 10 includes a shift register 10a which temporarily stores the image data signal for each pixel, and a data latch and D/A 10b which latches image data signals of one horizontal line stored in the shift register 10a, simultaneously D/A converts the data of one horizontal line, and outputs the data. A region in which a plurality of the pixel sections 14 are arranged in a matrix form is shown in the figure as an effective pixel region 18 of the display panel, and a display is achieved in the effective pixel region 18 based on the image data signal.
Although the resistance component due to the electrical wiring is not shown in the pixel circuit of FIG. 1, because a plurality of pixels are connected to the PVDD line as shown in FIG. 2, if there is a resistance component, the voltage of the source of the transistor (TFT 1) which drives the organic EL element would vary depending on the magnitude of the current of the other pixels. In the example structure of FIG. 2, the PVDD line is placed for each column of pixels, and as the current to the pixels connected to the same PVDD line is increased, the voltage drop is also increased.
If the reduction in the source voltage of the driving TFT 1 occurs when the selection TFT 2 is switched ON and a data voltage is written to the storage capacitor C, because the data voltage written to the gate does not vary, the absolute value of Vgs of the driving TFT 1 is reduced, resulting in a reduction in the current in the driving TFT 1, a reduction in the current of the organic EL element 3, and a reduction in the light emission brightness. In order to solve this problem, in U.S. Patent Application Publication No. 2007/0128583, a transistor which switches the current of the pixel OFF during writing of data is added in order to prevent the voltage drop of the horizontal line.
FIG. 5 is a diagram showing a voltage drop when the whole of a panel on which a power supply line is provided along the horizontal direction in parallel to the pixels is switched ON. In this example configuration, vertical PVDD lines are provided on both sides of the display panel, and a power supply voltage PVDD is supplied from the outside through the PVDD terminal to the vertical PVDD line. When the voltage at a center of the pair of vertical PVDD lines is V1, a voltage at the center portion at the upper end and the lower end of the panel is V2, a voltage at the center portion of the panel is V3, a horizontal direction at the center in the vertical direction of the panel is x-x′, and a vertical direction at the center of the horizontal direction of the panel is y-y′, the voltage of the horizontal PVDD line is low at the center portion for x-x′ and also for y-y′.
As described, when a current flows through a power supply line having a resistance component, a power supply voltage of the pixel circuit is reduced and the display brightness becomes uneven. For example, in a panel in which a power supply line is placed as shown in FIG. 5, when a white window pattern is displayed on a gray background, the left and right (portions b and c) of the window becomes darker than the other background portions (portions d and e) as the distance to the window is reduced, and a boundary with other portions becomes more noticeable, as shown in FIG. 6. In addition, if a transistor is added to the pixel circuit as a countermeasure as in Patent Document 1, the aperture ratio can be reduced or a percentage of failure can be increased, and thus a countermeasure is desired which does not require the addition of a transistor to the pixel circuit.
In accordance with the present invention, there is provided, in an electroluminescent display device having a plurality of pixels arranged in a plurality of rows and one or more columns and having for each row a respective gate line placed along a horizontal direction, wherein each pixel includes a selection thin film transistor (TFT) and a driving TFT each having respective first, second and gate electrodes, and an electroluminescence (EL) element, wherein the second electrode of the selection TFT is connected to the gate electrode of the driving TFT and the second electrode of the driving TFT is connected to the EL element, and wherein each gate line is connected to the respective gate electrodes of the selection TFTs of the pixels in the corresponding row, the improvement comprising:
(a) a first and a second power supply;
(b) a respective power supply line for each row, wherein each power supply line is placed along a horizontal direction and is connected to the respective first electrodes of the driving TFTs of the pixels in the corresponding row;
(c) a plurality of switches, each connected to one or more power supply lines, for selectively connecting the corresponding one or more power supply lines to either the first or the second power supply;
(d) a gate driver for selecting a gate line; and
(e) a selecting circuit for controlling the plurality of switches, wherein the selecting circuit causes the power supply line corresponding to the selected gate line to be connected to the first power supply, and the one or more power supply lines not corresponding to the selected gate line to be connected to the second power supply.
According to one aspect of the present invention, there is provided an active matrix display device having a self-emissive element of a current-driven type in each of a plurality of pixels arranged in a matrix form and in which a display is achieved by controlling a current of the self-emissive element, the active matrix display device including a gate line which is placed along a horizontal direction and which switches a thin film transistor for supplying data to pixels of a corresponding horizontal line ON and OFF, a horizontal power supply line which is placed along a horizontal direction and which supplies a current to pixels of a corresponding horizontal line, and a switch wherein the horizontal power supply lines are divided into groups, each including one or a plurality of the horizontal power supply lines, and the switch connects, in a switching manner, the group of the horizontal power supply lines to at least two power supplies, wherein, with the switch, different power supplies are used for a power supply connected to the horizontal power supply line of a group to which a horizontal line selected by the gate line belongs and for a power supply connected to a horizontal power supply line of a group which does not include the horizontal line selected by the gate line.
According to another aspect of the present invention, it is preferable that, in the active matrix display device, the group includes a plurality of horizontal power supply lines, the active matrix display device includes a connecting section which is provided on one side or on both sides of the horizontal power supply line and which connects a plurality of horizontal power supply lines within a group, and the switch connects the connecting section to at least two power supplies in a switching manner.
According to another aspect of the present invention, it is preferable that, in the active matrix display device, the group includes one horizontal power supply line, and the switch connects each horizontal power supply line to at least two power supplies in a switching manner.
According to another aspect of the present invention, it is preferable that, in the active matrix display device, switching of the switch is controlled by the gate line.
According to another aspect of the present invention, it is preferable that, in the active matrix display device, as a power supply of a group to which a horizontal line selected by the gate line belongs, a voltage which is lower than a voltage of a power supply of other groups is used.
According to another aspect of the present invention, it is preferable that, in the active matrix display device, the self-emissive element of current-driven type is an organic electroluminescence element.
The present invention can inhibit a phenomenon in which a power supply voltage of a pixel circuit is reduced during writing of data, the data to be written varies, and the display brightness becomes uneven.